Field of the Invention
The present invention relates generally to a pneumatic trip system for a steam turbine and, in particular, to a pneumatic trip system that includes an actuator assembly that exercises the trip valve via a pneumatic circuit without requiring a shutdown of the steam turbine.
Description of Related Art
A trip valve for a steam turbine is a safety device that must close quickly to shut off a flow of steam to the turbine during an emergency situation. Turbines typically run for long periods of time without shutting down, so the trip valve will typically remain in an open position for an extended period of time. When the trip valve remains in the open position for an extended period of time, scale and corrosion often build up between the valve stem and bushings in the steam chest trip valve assembly. This scale and corrosion may bind up the trip valve and prevent proper operation during an emergency situation.
Pneumatic trip systems provide a vital safety function for steam turbines. Proper maintenance and testing are necessary to ensure safe and reliable equipment operation. However, many mechanical trip systems are not tested as frequently as recommended, if at all.
Typically, trip valves require a complete closure, i.e., a complete valve actuation between open and closed positions, to determine if the trip valve is operational. This complete closure requires the turbine and driven equipment to be completely shut down, thereby disrupting the operation of the steam turbine. An available RCS (Redundant Control System) provides a means to exercise solenoid valves, but the RCS does not exercise a trip valve.